Excrutiating situation. Whether or not this is a question from personal experience or a hypothetical situation, my answer would be the same. If no one knows at this point, just calm down and think. Do a Google search for Dr. Scott Halzman who writes about recovering from infidelity. This conundrum would be called a Cornelian Dilemma I believe, (a dilemma in which someone is obliged to choose between two courses of action either of which will have a detrimental effect on themselves or on someone near to them.) I was in an ALMOST similar situation and handled it poorly and lost everything I held dear - most importantly, the psychological health of my daughter. 1 I wish I had NEVER told ANYONE I knew. 2. I wish I could have explored the psychology of what was going on. I think people who commit such an obviously egregious act against societal norms, against what anyone in most religions would call moral, against all manner of kindness, good sense, and thought of the future - people who would do this are acting out of some deep-seated psychological issue that has little to do with the spouse and not to get all fruedian here, probably has to do with mother-son, daughter-father relationships. I would also guess, playing armchair psychologist, one or both of the people has issues with authority. So, the offended brother probably has always realized that one or both of these people in his life is a bit "off" in terms of psychological health and has to be the strong, balanced person dealing with the situation. Why? Because EVERYONE in this situation is at risk of losing EVERYTHING that is of value. So counseling by the RIGHT person for the offended brother is a first step. Many counselors will view my perspective as unhealthy, so you have to find one that feels that there is hope for recovery from this terrible dilemma. I don't feel this is an unhealthy position because I know what I lost. Yes, it would have taken years to get over the infidelity and for my husband and me to restore the marriage. Instead I have NEVER gotten over the loss of my family and the tremendous cost my daughter had to pay is THE sorrow of my life, THE regret. Am I happy now? Mostly Did I recover? about 80 % Do I wish, after 2 years of trying to get my marriage back, that I had tried one month, two months, three months, a half of year longer, maybe even another year longer? Yes.